The present invention relates to a process for producing an injection laser and to a laser obtained by this process. It is intended more particularly for use in optical telecommunications.
The injection laser according to the invention is of the double heterostructure type. Such a structure comprises a monocrystalline substrate in which are deposited by liquid phase epitaxy four layers of semiconductor materials, successively constituted by:
a first n-doped confinement layer, PA0 a second active layer, generally based on p-doped gallium arsenide, PA0 a third p-doped confinement layer, PA0 a fourth p.sup.+ -doped contact layer. PA0 a first confinement layer of composition Ga.sub.1-x Al.sub.x As in which x is a number below 1 (generally between 0.3 and 0.5) said layer being n-doped, PA0 a second active layer of composition Ga.sub.1-y Al.sub.y As (y being generally a number below 0.1), said layer generally being p-doped, PA0 a third confinement layer of composition Ga.sub.1-z Al.sub.z As, (z being a number below 1) said layer also being p-doped; PA0 finally, a fourth contact layer of composition GaAs and p.sup.+ -doped. PA0 (a) In the first category, the optical beam is guided by the amplification gain profile (lasers guided by the gain), such lasers being of the type most frequently encountered. The technology used may call on: PA0 (b) In this second category, the optical beam is guided by a refractive index gradient "Lasers guided by the index". Generally this consists of an index step which is higher in the stripe than outside it. Compared with lasers guided by the gain, this type of laser has a better linearity of the optical power emitted as a function of the current. Numerous laser structures guided by the index have been described in the literature, reference being made e.g. to the following: PA0 (A) A conventional double heterostructure is formed (n-type substrate, n-type first confinement layer, active layer, second confinement layer and P.sup.+ -type contact layer), but the active layer is of the weakly doped n-type; PA0 (B) a masking layer is deposited on the contact layer and its composition is similar to that of the confinement layers; PA0 (C) the latter layer is chemically etched so as to only leave behind a masking "mesa"; PA0 (D) zinc diffusion is performed using said "mesa" as the mask, the diffusion time being sufficiently great for the diffusion to affect the active zone; PA0 (E) there is proton bombardment of the structure by once again using the "mesa" as the mask, the bombarded zone also extending beyond the active zone; PA0 (F) the "mesa" is eliminated.
According to a known procedure, the surface of the optically active zone is reduce by limiting it to a thin stripe obtained by making part of the third and fourth layers electrically neutral. This neutralization is brought about by subjecting the structure previously covered with a mask corresponding to the desired stripe to a proton bombardment.
In particular, lasers of this type are known which emit at about 0.85 .mu.m and which comprise:
For more details on these known structures and the materials used, reference can be made, for example, to the article by J. P. NOBLANC entitled "Fibre optical communications devices" published in the Journal "Applied Physics", 13, pp. 221-223, 1977 or in the work by H. KRESSEL and J. K. BUTLER entitled "Semiconductor laser and heterojunction leds" published by Academic Press.
The stripe structure makes it possible to limit the extent of the electrically excited region to a rectangular surface only a few microns wide (approximately 10 .mu.m) and a few hundred microns long (300 to 500.mu.). The threshold currents for the appearance of laser emission then become compatible with continuous operation at ambient temperature.
Numerous procedures have been developed for forming such stripes and they can be classified into two categories;
photon bombardment as described in the article entitled PA1 the opening of a window in the contact layer by means of a layer of silica, as described in the article entitled "Advances in GaAs junction lasers with stripe geometry", published by L. D. D'ASARO in the "Journal of Luminescense", Vol. 7, pp. 310-337, 1973; PA1 a selective diffusion of zinc, as described in the article entitled "A GaAs-Al.sub.x Ga.sub.1-x As Double Heterostructure Planar Stripe Lasers" by H. YONEZU et al, published in the Journal "Japanese Journal of Applied Phsyics", Vol. 12, No. 10, pp. 1585-1592, October 1973. PA1 the laser with a channel substrate planar (C.S.P) structure described in the article entitled "Channel substrate planar structure (AlGa)As injection lasers" by K. AIKI et al and published in the Journal "applied physics letters", Vol. 30, No. 12, pp. 649-651, June 16th 1977 in which the indexed profile is obtained by forming in the substrate a channel with a width of a few microns and the type n confinement layer then has a variable thickness which induces an index jump level with the channel; PA1 the laser with a selective zinc diffusion in the active region which brings about an index step by electronic compensation of the impurities present and as described in the article entitled "New stripe geometry laser with high quality lasing characteristics by horizontal transverse mode stabilization-a refractive index guiding with Zn doping" by H. YONEZU et al, published in "Japanese Journal of Applied Physics", Vol. 16, No. 1, pp. 209-212, 1977.
"Optical and electrical properties of proton bombardment of p-type GaAs" published by J. C. DYMENT et al in the Journal "Journal of Applied Physics", Vol. 44, No. 1, January 1973, pp. 207-213;
The "buried" laser where the active GaAs part is completely surrounded by GaAlAs with a lower optical index than GaAs and as described in the article entitled "GaAs-Ga.sub.1-x Al.sub.x As buried-heterstructure injection lasers" published by T. TSUKADA in the "Journal of Applied Physics", Vol. 45, No. 11, pp. 4899-4906, November 1974.
Finally, the laser described in the article by J. C. Bouley et al and entitled "A stabilized zinc diffused, proton bombarded (GaAl)As laser" published in IEEE Quantum Electronics, August 1979, this laser being guided by the index in which an index step is obtained by zinc diffusion, said diffusion affecting the sides of the stripe.
Lasers with an index profile all lead to a certain number of technological difficulties. These difficulties may be due to the need of carrying out liquid epitaxy in two stages (case of the laser with a buried structure) or through it being necessary to accurately control the diffusion depth (Case of the laser with "selective diffusion") or due to obtaining an alignment of optical stripe and the electric stripe where pumping takes place (Case of the C.S.P laser).
The structure described in the article by J. C. BOULEY et al has the disadvantage of having a mesa above the stripe. Therefore, this structure is not planar, which leads to problems when welding the system to its base. In addition, technological difficulties occur during the alignment of non-diffused and non-bombarded stripes.